In the production of filled paper, an aqueous suspension containing cellulosic fibres, fillers and additives, referred to as the stock, is fed into a headbox which ejects the stock onto a forming wire. Water is drained from the stock through the forming wire so that a wet web of paper is formed on the wire, and the web is further dewatered and dried in the drying section of the paper machine.
Highly filled paper is an established trend in the paper industry, not only due to the savings in the decreased use of fibre, but also due to improved product quality, such as higher opacity and better printability. For super calandered paper (SC paper) and many paper grades containing mechanical fibres, kaolin clay is the most commonly used filler. The clay particles have a flake or plate shape, and at calandering the flakes are uniformly oriented, giving a high gloss and smoothness to the paper. The amount of filler can be as high as 30% or more.
A high degree of filler causes a decrease in paper strength as well as linting and dusting. A rule of thumb at filler usage in paper is that a 10% increase in filler content decreases the strength by 20%. Problems with linting and dusting occur, as small fibre fragments and fillers are not properly bound into the paper. This can give a specific problem at rotogravure printing of SC paper, called missing dots, when ink is missing in dots in the print.
Addition of a binding agent can increase the strength of the paper as well as decrease the linting and dusting. Among other materials starch has been used as a binding agent. However, in SC paper making, the calandering of the paper is done at a load of 100-350 kN/m. Starch makes the paper brittle and it can break at such heavy loads. Starch also makes the paper denser already prior to calandering. Therefore, no starch or small additions (1-2 kg/tonne dry paper) is used in SC paper making.
Conventional CMC is sometimes added to the wet end as a strength additive. However, then the problem is that dewatering is slowed down considerably. A third possibility, synthetic strength additives, can be used but they are often quite expensive.
For all paper grades with high filler loadings, paper strength, drainage and filler retention are important issues. Sometimes size is added, and then also size consumption is higher when the filler loadings are increased and/or when starch is not used.
U.S. Pat. No. 5,759,346 describes a method of improving strength and reducing lint and dust in the production of tissue paper. The filler is a kaolin clay which has been pre-treated with a cationic starch.
WO 01/86067 describes a method for pre-treating a filler with a hydrophobic polymer, which is a synthetic polymer comprising acrylate and styrene monomers. The use of the pre-treated filler improves wet strength and reduces the linting of the paper.
Furthermore, WO 95/13324 refers to calcium carbonate treated with a cellulose derivative such as sodium carboxymethyl cellulose (“CMC”) having a degree of substitution of 0.70. Said treated calcium carbonate is used as filler in alkaline papermaking suspensions whereby the brightness of the paper is increased.
There is still need for a filler which provides an improved papermaking process and better properties of the paper produced. It would be desired to provide a filler which renders possible production of highly filled paper showing excellent printing and mechanical properties. It would also be desirable to provide a filler which is compatible with drainage and retention aids, and hereby leads to good drainage, retention and paper machine runnability. It would also be desirable to provide a simple and efficient process for producing a filler showing the above characteristics.